The invention relates to a method for controlling active power flow in a high voltage direct current transmission system having a first and a second converter station coupled to each other via a direct current link, each converter station with a voltage source converter, wherein the first converter station controls the voltage of the direct current link, at the first converter station, in dependence on a first voltage reference value, and the second converter station controls the active power flow through the second converter station, and to a high voltage direct current transmission system for carrying out the method.
For a general description of control systems for voltage source converters reference is made to Anders Lindberg: PWM and Control of Two and Three Level High Power Voltage Source Converters. Royal Institute of Technology, Department of Electric Power Engineering. Stockholm 1995, in particular pages 1, 77-104, and appendix A.
FIG. 1 shows in the form of a schematic single line and block diagram a high voltage direct current transmission system as known in the prior art. A first and a second converter station STN1 and STN2 respectively, are coupled to each other via a direct current (dc) link having two pole conductors W1 and W2 respectively. The pole conductors are typically cables but may also at least to a part be in the form of overhead lines. Each converter station has a capacitor equipment, in this embodiment schematically shown as capacitors C1 and C2 respectively, coupled between the pole conductors, and comprises a voltage source converter, CON1 and CON2 respectively. Each converter comprises two three-phase groups of semiconductor valves in six-pulse bridge connections The semiconductor valves comprises, in a way known per se, branche of gate turn on/turn off semiconductor elements, for example power transistors of so-called IGBT-type, and diodes in anti-parallel connection with these elements.
Each converter is via phase inductors, PI1 and PI2 respectively, coupled to a respective three-phase alternating current (ac) electric power network, N1 and N2. Although not shown in the figure, it is well known in the art that the converters may be coupled to the three-phase networks via transformers, in which case the phase inductors in some cases may be omitted. Filter equipment F1 and F2 respectively is coupled in shunt connection at connection points between the phase inductors and the three-phase networks.
The ac-voltage of the alternating current network N1 at the connection point of the filter F1 is designated UL1 and is sensed with a sensing device M1. The ac-current at the converter CON1 is designated Iv1 and is sensed with a measuring device M2. Similarly, the ac-voltage at the connection point of the filter F2 is designated UL2 and is sensed with a sensing device M3, and the ac-current at the converter CON2 is designated Iv2 and is sensed with a measuring device M4.
The dc-voltage across the capacitor equipment C1 is designated Udc1 and is sensed with an only symbolically shown sensing device M5, and the dc-voltage across the capacitor equipment C2 is designated Udc2 and is sensed with an only symbolically shown sensing device M6.
The first converter station comprises control equipment CTRL1 and the second converter station control equipment CTRL2 of similar kind.
The control equipments operate in a conventional way with three phase units (voltages and currents) converted to and expressed in a two-phase xcex1xcex2-reference frame as well as in a rotating two-phase dq-reference frame. The phases of the three-phase alternating current electric power networks are referred to as the abc-reference frame. Vector units are in the following illustrated with a dash on top ({overscore (x)}). In the following text and in the figures the reference frame is, where appropriate, indicated in an upper index (for example xdq).
Control equipment CTRL1 comprises a dc-voltage controller UdcREG, an ac-voltage controller UacREG, selector means SW1 and SW2 respectively, and an internal converter current control IREG.
The dc-voltage controller is supplied with the sensed dc-voltage Udc1 and a first voltage reference value Udc1R thereof, and forms in dependence of the deviation of the actual value Udc1 and the first voltage reference value Udc1R an output signal P1C.
The ac-voltage controller is supplied with the sensed ac-voltage UL1 and a voltage reference value UL1R thereof, and forms in dependence of the deviation of the actual value UL1 and the reference value UL1R an output signal Q1C.
Each of the dc-voltage controller and ac-voltage controller comprises a (not shown) difference forming member, forming the deviation between respective reference values and actual values, which deviation is supplied to and processed in a (not shown) controller member having for example a proportional/integrating characteristic. The voltage controllers thus provide feedback control of the respective voltages.
The output signal P1C and a reference value P1R for the active power flow through the converter CON1 are supplied to two different inputs on the selector means SW1, and the output signal Q1C and a reference value Q1R for the reactive power flow through the converter CON1 are supplied to two different inputs on the selector means SW2. The reference values P1R and Q1R may be set manually, in particular the reference value P1R may also be the output of another controller such as a frequency controller.
In dependence on a first mode signal MD11 either of the output signal P1C and the reference value P1R is transferred and supplied to the internal converter current control IREG in the form of a signal designated pref1, having the significance of an active power order.
In dependence on a second mode signal MD21 either of the output signal Q1C and the reference value Q1R is transferred and supplied to the internal converter current control IREG in the form of a signal designated qref1, having the significance of a reactive power order.
Thus, each converter station can operate in four different modes, one of dc-voltage control and active power control and one of ac-voltage control and reactive power control. Usually, one of the converter stations, for example the first one, operates under dc-voltage control, whereas the second converter station (as well as other, not shown, converter stations, which may be coupled to the first converter station via other direct current links) operates under active power control and under ac-voltage or reactive power control.
The operation modes are set either manually by an operator, or, under certain conditions, automatically by a not shown sequential control system.
The internal converter current control IREG is of a kind known per se and comprises a current-order calculating unit and a converter control unit (not shown).
The current-order calculating unit comprises a current-order calculating member and a current limiting member. The above mentioned active and reactive power order signals, pref1 and qref1 respectively, are supplied to the current-order calculating unit. In the current-order calculating member current reference values, expressed in the dq-reference frame as irefd and irefq respective, are calculated in dependence on the power orders. The calculation is performed according to the per se known relations
pref=udirefd+uqirefq
qref=udirefqxe2x88x92uqirefd
wherein the voltages ud and uq represent voltages sensed in the alternating current network and transformed to the dq-reference frame in a manner known per se. The current reference values irefd and irefq are supplied to the current limiting member and therein limited, as the case may be, in accordance with specified operating conditions for the transmission system. The current limiting member outputs the so limited values as a current vector {overscore (i)}refxdq to the converter control unit.
The converter control unit has an inner ac-current control feed back loop which, in dependence on the supplied current vector {overscore (i)}refxdq and a phase reference signal, generates a voltage reference vector. This voltage reference vector is supplied to a pulse-generating member that in dependence thereon generates a train Fp1 of turn on/turn off orders supplied to the semiconductor valves according to a predetermined pulse width modulation pattern. The phase reference signal is in a conventional manner generated by a phase locked loop and at least under steady state conditions locked to the phase of the filter bus voltage of the alternating current electric power network.
Control equipment CTRL2 in the second converter station is similar to control equipment CTRL1 described above, only, in FIG. 1, index 1 for the various signals is at appropriate occasions changed to index 2.
As mentioned above, usually one of the converter stations, for example the first one, operates under dc-voltage control, controlling the dc-voltage of the dc-link at that converter station, whereas the second converter station operates under active power control and under ac-voltage or reactive power control. The converter station operating under dc-voltage control then has an active power slack function, providing the active power requested by the second converter station and maintaining the dc-voltage at the desired value. However, certain disturbances in the power networks and in the transmission system, in particular transient disturbances in the alternating current electric power network to which the dc-voltage controlling converter station is coupled, for example phase to ground faults, may result in considerable dc-voltage variations on the direct current link. In such cases, the first converter station may not be able to balance the active power required by the second converter station. Such variations may reach such a magnitude that they would lead to a shut down of the dc transmission system, such as a temporary blocking of the converter station by an over current protection or by an over- or under voltage protection, if no measures were taken. One such measure is to temporarily change the operating mode of the second converter station to dc-voltage control mode in order to keep the dc-transmission system in operation. When the disturbance is cleared, the operating mode of the second converter should then again be changed to active power control mode in order to recover the active power flow to the pre-fault level. Such mode changes to dc-voltage control mode have to be done very fast in order to avoid dc over voltages on the dc-link. Furthermore, to determine the time at which a change back to active power control mode may be accomplished requires in practice intervention of the operator of the dc transmission system and communication between the converter stations.
The object of the invention is to provide a method of the kind described in the introduction, which eliminates the above mentioned disadvantages related to change of operating modes as known in the prior art, and a high voltage direct current transmission system for carrying out the method. In particular it is an object of the invention to provide a method, and a high voltage direct current transmission system, which permit a smooth transition between dc-voltage control and active power control modes of the converter station.
According to the invention, this object is accomplished by having both the first and the second converter stations operating in dc voltage control mode, the first converter station having means for control of the voltage of the direct current link in dependence on a first voltage reference value and the second converter station having means for control of the active power flow through the second converter station, which means comprises voltage control means for control of the direct voltage of the second converter station in dependence on a second voltage reference value, said second voltage reference value being formed in dependence on a third voltage reference value and on a voltage reference correction signal formed in dependence on a quantity indicative of the active power flow through the second converter station and a reference value thereof.
In an advantageous development of the invention, the means for control of the active power flow through the second converter station comprises controller means for forming said voltage reference correction signal, having as inputs said quantity indicative of the active power flow through the second converter station and said reference signal thereof, and summing means for forming said second voltage reference value as a sum of said voltage reference correction signal and said third voltage reference value.
In another advantageous development of the invention, said quantity indicative of the active power flow through the second converter station is the active power flow through the second converter station, calculated from measurements of voltages and currents in the alternating current network to which the second converter station is coupled.
Other advantageous developments of the invention will become clear from the following description and from the claims.
With the invention the dc-voltage variations in the transmission system during a disturbance will be reduced and the power recovery process after a disturbance will be simplified and faster. In particular the need for a communication system between the converter stations may be eliminated, such systems being quite complicated especially in multi-terminal systems.